This week's Fiona Bruce documentary was a revealing example of one of the most sensitive issues in factual television: the tension between access and editorial control.

Why was Victoria: a Royal Love Story (BBC1) screened last Sunday night? TV schedules are often shaped by anniversaries, but there were no key Victorian dates close to March 14. As Bruce's opening voiceover acknowledged, the film was going out now because the love-gifts of the Royal couple are the subject of "an exhibition in the Queen's Art Gallery at Buckingham Palace".

This is a standard devil's deal between broadcasters and institutions. The Royal galleries only allowed filming in return for publicity for the show. The curators cannily ensured that many of the artworks were shown being installed.

But the film-makers did try as hard as possible to make a free-standing historical documentary. Interestingly, Bruce's references to "an exhibition" were coy and sparing: in the past, there might have been cutaways of posters or even details of dates. (The Queen's Gallery is said to have privately expressed regret that more was not made of the links between show and programme.)

The reason for such discretion is that the BBC is increasingly nervous about seeming to give free advertising, after receiving complaints about the boost to Andrew Lloyd-Webber and Cameron Mackintosh productions from Graham Norton's audition shows.

One consequence is that films inspired by artistic blockbusters are now rarely shown close to the opening: next week's Culture Show Special (BBC2) on Henry Moore comes a month after the linked exhibition started at Tate Britain.

But the problem is a wider one. Inside John Lewis (BBC2) is an absorbing insight into the retail industry, but will give a boost to the shop's profile that would be vastly expensive if purchased in the form of TV commercials.

Documentary is impossible without access, but the timing of admission and transmiss ion is always delicate.